The surface plasmon is a particular kind of electromagnetic wave which propagates along the surface of a metal (H. Raether, "Surface plasmons on smooth and rough surfaces and on gratings", Springer-Verlag, Berlin, 1988). Optical excitation of the surface plasmon can be achieved if a p-polarized, collimated light beam undergoes total reflection on the surface of glass substrate coated with a thin metal film (so-called Kretschmann configuration). The momentum of photons should match the surface plasmons on the opposite surface of the metal film in order to make this possible. This occurs for a given wavelength at a given critical angle of incidence of light. The phenomenon is observed as a sharp minimum in the intensity of the reflected light when the angle of incidence is varied. The angle or wavelength at which this dip occurs depends decisively on the properties of the surface layer on top of the metal film, and therefore the phenomenon can be used to monitor changes on this surface layer caused e.g. by a specific chemical or biological reaction or by the change of concentration of some substance in the immediate vicinity of this surface.
In principle, any material having a negative dielectric function can be used for the excitation of plasmons Most metals fulfill this requirement in the range of visible wavelengths. An ideal material (SPR-material) should have a negative coefficient of the real part of the dielectric constant and as large an absolute value of this coefficient as possible and at the same time as small an absolute value of the coefficient of the imaginary part as possible. In the previous methods utilizing the SPR-phenomenon the choice of metals and their thicknesses has been made usually to satisfy the criterion for the sharpest peak with as total an extinction of the intensity of reflected light as possible. For this reason the metals most used are silver and gold, silver giving the sharpest peaks. Because of the steep curve the sensitivity of the method is good when a constant angle of incidence is used. When the aim is to improve the dynamics of the method, several angles of incidence can be used additionally for carrying out a simultaneous intensity measurement. The method of the above kind is disclosed for example in GB-Patent Application 2,197,065. It is also possible to observe the location of the peak, that is, the measurement gives more data which are independent of each other.
The SPR-material is in addition coated with a specific surface layer having certain affinity towards the test material to attain desired sensitivity. The changes occurring in this layer form the basis for the measurement.
The purpose of the invention is to present an improvement for the above-mentioned methods and to present a method and sensor making it possible to widen the field of use of the SPR-method. When a catalytically active material is used as the material, the catalytic properties and the SPR-properties of the material can be combined in a way that creates new possibilities of analysis.
The method and sensor can also make use of materials having a relatively large optical absorption. This kind of material, even if it creates a broad peak, can be used in SPR-sensors on the prerequisite that the analyses are carried out at an angle of incidence and wavelength that are properly chosen.